I would imagine that you are trying to draw the grid that you are using with the spatial data you are displaying. You currently need to have a layer for something like this, with the spatial data in a data source, like a shapefile.

I'm building pluggable renderers into v2.0, and I'd been trying to think of other use cases for a custom renderer... this is an excellent candidate.

I strongly support your effort for pluggable renderers. One question... would these renderers get called for each feature in a dataset, or would there be a single call and the render would draw all features at once? This latter functionality is important
to me and I could assign a development resource for design and/or testing feedback.